Heretofore, exhaust pollution control devices for internal combustion engines were provided which controlled emissions of carbon monoxide and hydrocarbons, but not oxides of nitrogen. Control of carbon monoxide and hydrocarbons was accomplished by increasing the operating temperature of the engines, which allowed more complete combustion of fuel. However, increasing the combustion temperature of internal combustion engines increases the production of nitrogen oxides in the exhaust. As a result, the engines are capable of emitting low concentrations of carbon monoxide and hydrocarbons in the exhaust, but high concentrations of nitrogen oxide are still present.
Examples of prior art internal combustion engines are U.S. Pat. No. 3,141,293 to Crooks; U.S. Pat. No. 2,877,622 to Antonissen; U.S. Pat. No. 2,703,560 to Lieberherr; U.S. Pat. No. 2,703,561 to Froehlich; U.S. Pat. No. 2,766,744 to Steiger; U.S. Pat. No. 2,645,409 to Lawler; U.S. Pat. No. 3,483,854 to Foran et al; U.S. Pat. No. 2,994,187 to Kelgard; U.S. Pat. No. 2,571,256 to King; and U.S. Pat. No. 2,678,532 to Miller.